Reliability of a power system is considerably influenced by its equipments.
Power transformers are one of the most critical and expensive equipments of a power
system and their proper functions are vital for the substations and utilities. Therefore,
reliability model of power transformer is very important in the risk assessment of the
engineering systems. This model shows the characteristics and functions of a transformer in
the power system. In this paper the reliability model of the power transformer with ONAN
cooling is obtained. The transformer is classified into two subsystems. Reliability model of
each subsystem is achieved. Markov process representation and the frequency/ duration
approach are employed to obtain a complete reliability model of the subsystems. By
combining these models reliability model of power transformer is obtained. The reliability
model associated with the transformer is then proposed combining the models of
subsystems. The proposed model contains five states. To make the model more applicable,
the 5-state model is alleviated to a 3-state one. Numerical analysis and sensitivity analysis
relevant to the proposed reliability model are performed for evaluating the numerical values
of the model parameters and the impact of different components on the reliability of the
model.